Abstract DNA in the nucleus of eukaryotic cells is packaged into the higher-order chromatin structure by histone proteins. Thus, DNA templated cellular processes require alteration of chromatin structure to dynamically facilitate access to packaged DNA. In general, this is accomplished by ATP-dependent chromatin remodeling, histone exchange or chemical modification of histone proteins. Phosphorylation of serine/threonine residues in particular has been shown to function in a number of cellular processes, including transcription, mitosis, apoptosis, and sporulation. We have isolated a novel histone kinase complex in yeast containing conserved S-phase regulatory proteins. This complex is capable of phosphorylating free histones, but targets histone H3 in the context of the intact histone octamer. Our preliminary data indicate that phosphorylation maps to a previously undescribed site of histone modification that may play a critical role in histone-DNA interactions and is likely of great significance to the process of DNA replication. The goals of this proposal are to purify and fully characterize the components of the multi-protein histone kinase activity, to characterize the function of this novel epigenetic mark in vivo and verify its dependence on the kinase complex in question. We also propose to map this modification on 1% of the human genome to assess its stability, localization and its suitability as a new epigenetic marker of replicating and proliferating cell types. Given our hypothesis that this modification functions in licensing of replication, we will also compare its relative abundance across the cell cycle and its requirement for cell cycle progression. As uncontrolled cell division and DNA replication is associated with the proliferation of cancer cells we anticipate that this epigenetic mark will provide a biomarker of diseased states and will offer a novel target for cancer therapeutics.